1. Field of the Invention
This invention relates to a method for making a graphite film or sheet or block which is adapted for use in X-ray optical elements, X-ray monochromators, neutron ray diffraction mirrors or filters. The invention also relates to radiation optical elements using the graphite film.
2. Description of the Prior Art
Graphite is very important as an industrial material because of its remarkably high resistances to heat and chemicals and high conductivity. It has accordingly wide utility in the fields of electrodes, heating elements and other structural materials. Among various types of graphite, artificial graphite having similar characteristics to single crystal graphite has been widely employed as a monochromator or filter for X-rays or neutral rays or as a spectral crystal. This is because the artificial graphite has good spectral and reflective characteristics with respect to the X-rays or neutron rays.
As a matter of course, single crystal graphite may naturally occur but high quality graphite is limited in amount of production. In addition, naturally occurring graphite is obtained in the form of powder or small blocks and is thus difficult to handle. Accordingly, such graphite has often been prepared artificially.
The preparation of the artificial graphite can be broadly classified into the following two groups.
The first group includes sedimentation from Fe, Ni/C melts, decomposition of carbides of Si, Al and the like, or cooling of a carbon melt under high temperature and high pressure conditions. The graphite obtained from this group has substantially the same physical properties as natural graphite called kish graphite. However, the graphite obtained according to the methods of the first group is in the form of fine flakes. Additionally, the methods are complicated, so that the production costs becomes high. Thus, the methods of this group are rarely used industrially at present.
The second group includes a method in which a hydrocarbon gas is pyrogenically deposited in a gas phase and hot processed. The method includes a further step of re-annealing the deposited product under a pressure at a temperature of 3400.degree. C. for a long time. The resultant graphite is called highly oriented pyrographite (HOPG) and exhibits characteristics similar to those of natural graphite. The graphite obtained by this method has a larger size than the kish graphite. However, the method is also complicated with a low yield, thus leading to high production costs.
On the other hand, graphitization of various organic matters or carbonaceous materials has been long considered in which the organic matters or carbonaceous materials are heated over 3000.degree. C. However, this method has not been successful in obtaining graphite whose physical properties are similar to those of natural or kish graphite. For instance, the electric conductivity along the direction of ab planes which is one of typical physical properties is 1 to 2.5.times.10.sup.4 S/cm for natural or kish graphite. The conductivity of the graphite obtained by the above method is usually as low as 1 to 2.times.10.sup.3 S/cm. This means that the graphitization does not completely proceed when organic matters or carbonaceous materials are treated by the method.
In the graphitization method, it is ordinary to use as starting materials a carbonaceous material such as coke and coal tar as a binder. When coke or charcoal is heated to about 3000.degree. C., the resultant carbon has a number of structures including a structure relatively close to the structure of graphite and a structure which is completely different from the graphite structure. Carbon which relatively readily converts into a graphite structure simply by heat treatment is called ready-to-graphitize carbon. The other type of carbon is called hard-to-graphitize carbon. The reason why the difference in the structure is produced has close relation with a graphitization mechanism. More particularly, whether graphitization proceeds readily or not depends upon whether structural defects present in a carbon precursor are likely to be removed by the thermal treatment. This means that the fine structure of the carbon precursor plays an important role with respect to the graphitization.
In contrast to the method using coke and a binder as starting materials, several studies have been made on methods using polymer materials to form graphite films. In this method, use is made of the molecular structures of polymer materials and the fine structure of the resultant carbon precursor is properly controlled. The method includes thermal treatment of a polymer in vacuum or in an inert gas and formation of a carbonaceous product through decomposition and polycondensation reactions. However, it is known that high quality films of graphite cannot always be obtained using any polymers as starting materials, because almost all polymeric materials cannot be used for this purpose. For instance, a number of polymers have been thermally treated including phenol-formaldehyde resins, polyacrylonitrile, fibers of polyamides, poly-p-phenylene, poly-p-phenylene oxide, polyvinyl chloride and the like. These polymers have been found to be hard-to-graphitize materials, from which any substances having a high degree of graphitization have not been obtained yet.
We have made extensive studies on graphitization of a number of polymers. As a result, it has been found that when films of certain polymers are thermally treated at certain temperatures, they are more readily graphitized than known polymer materials. Such polymers include polyoxadiazole, aromatic polyimides, polybenzothiazoles, polybenzooxazole, polybenzobisoxazole, polythiazole, and films of aromatic polyamides. This graphitization is described in European Patent Application No. 0203581 and Japanese Laid-open Patent Application No. 61-275117.
According to the graphitization methods described above, the above-mentioned polymers are heated at 1800.degree. C. or higher, preferably at 2500.degree. C. or higher to obtain a graphite product having a high degree of graphitization within a relatively short time.
Although graphite products with good properties are obtained by the above methods, such methods have the following disadvantages.
One of the disadvantages resides in that a thick graphite sheet or block cannot be obtained. One may consider that the graphitization reaction is irrevalent with a thickness of a starting film. In fact, the reaction depends strongly upon the thickness of starting film. This is shown by the following experiment conducted by us. Four films of a polyoxadiazole with different thicknesses were graphitized to determine a lattice constant of the resultant graphite, a degree of graphitization and an electric conductivity along the direction of ab planes. The results are shown in Table 1 below.
TABLE 1 ______________________________________ Thickness of Degree of Conductivity Starting Treating Lattice Polymer- (along ab Film Temp. Constant ization planes) (micrometers) (.degree.C.) (angstroms) (%) (S/cm) ______________________________________ 5 2600 6.710 99 9800 25 2600 6.713 97 7800 100 2600 6.720 93 6100 450 2600 6.731 87 4900 ______________________________________
As will be apparent from the above results, the degree of the graphitization reaction varies with the thickness of the starting film. More particularly, the degree of graphitization varies from 99 to 87% depending upon the thickness of the film. This reveals that a thin film of graphite can be obtained from the polyoxidiazole but it is difficult to obtain a thick sheet or block of graphite.
The above fact is true of other polymers. For instance, when aromatic polyimide films having thicknesses of from 5 to 450 micrometers were graphitized, the degree of graphitization was found to be from 98 to 83%. For aromatic polyamide films, the degree of graphitization was from 99 to 88% when film thicknesses were from 10 to 600 micrometers.
The second disadvantage is that when polymer materials are merely heated, the resultant graphite is not improved in the characteristic of how to neatly superpose the planes along the ab axes of graphite on the direction of the c axis. This characteristic is important when the graphite is used as an optical element for X-ray. The degree of superposition of the ab planes is measured by a rocking method using X-ray diffraction. For using graphite crystals as an optical crystal element such as for X-rays, the rocking characteristic should generally be not larger than 0.4.degree. for a graphite film whose thickness is not larger than 50 micrometers, and not larger than 3.degree. for a 1 mm or thicker graphite block or sheet. The graphite films obtained from polyoxidiazole films indicated in Table 1 had, respectively, a rocking characteristic of 6.7.degree., 10.5.degree., 12.degree. and 17.degree. for the starting film thicknesses of 5, 25, 100 and 450 micrometers. This demonstrates that graphitization by mere heating cannot provide an X-ray element with a good rocking characteristic. Similarly, with graphite products obtained from aromatic polyimide films, the rocking characteristic was 8.degree., 11.degree., 14.degree. and 17.degree. for the thicknesses of the starting films of 5, 25, 100 and 450 micrometers, respectively. With graphite products obtained from aromatic polyamide films, the rocking characteristic was 7.5.degree., 9.5.degree., 11.degree. and 16.degree. for the thicknesses of the starting films of 10, 25, 100 and 600 micrometers. Thus, a satisfactory rocking characteristic cannot be obtained.
This is considered as follows: when the film becomes thicker, the ab planes are more unlikely to be oriented because of the generation of a gas from the inside of a sample accompanied by the thermal treatment.